Vitamin-targeted imaging agents

ABSTRACT

The invention relates to compounds and methods for targeting radionuclide-based imaging agents to cells having receptors for a vitamin, or vitamin receptor binding derivative or analog thereof, by using such a vitamin as the targeting ligand for the imaging agent. The invention provides a compound of the formula  
                 
 
for use in such methods. In the compound, V is a vitamin that is a substrate for receptor-mediated transmembrane transport in vivo, or a vitamin receptor binding derivative or analog thereof, L is a divalent linker, R is a side chain of an amino acid of the formula H 2 NCHRCOOH, M is a cation of a radionuclide, n is 1 or 0, K is 1 or 0, and the compound can be in a pharmaceutically acceptable carrier therefor. The vitamin-based compounds can be used to target radionuclides to cells, such as a variety of tumor cell types, for use in diagnostic imaging of the targeted cells.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of application Ser. No. 10/430,519,filed May 6, 2003, which claims priority under 35 U.S.C. §119(e) to U.S.Provisional Application Ser. No. 60/378,571, filed on May 6, 2002.

FIELD OF THE INVENTION

The invention relates to compounds and methods for targeting an imagingagent to cells of an animal. More particularly, radionuclide-basedimaging agents are targeted to cells having receptors for a vitamin byusing such a vitamin, or a vitamin receptor binding derivative or ananalog thereof, as the targeting ligand for the imaging agent.

BACKGROUND AND SUMMARY OF THE INVENTION

Transmembrane transport is a critical cellular function. Becausepractitioners have recognized the importance of transmembrane transportto many areas of medical and biological science, including drug therapyand gene transfer, there have been significant research efforts directedto the understanding and application of such processes. Thus, forexample, transmembrane delivery of nucleic acids has been attemptedthrough the use of protein carriers, antibody carriers, liposomaldelivery systems, electroporation, direct injection, cell fusion, viralcarriers, osmotic shock, and calcium-phosphate mediated transformation.However, many of those techniques are limited both by the types of cellsin which transmembrane transport occurs and by the conditions requiredfor successful transmembrane transport of exogenous molecules.Furthermore, many of these techniques are limited by the type and sizeof the exogenous molecule that can be transported across the cellmembrane without loss of bioactivity.

One mechanism for transmembrane transport of exogenous molecules havingwide applicability is receptor-mediated endocytosis. Advantageously,receptor-mediated endocytosis occurs both in vivo and in vitro.Receptor-mediated endocytosis involves the movement of ligands bound tomembrane receptors into the interior of an area bounded by the membranethrough invagination of the membrane. The process is initiated oractivated by the binding of a receptor-specific ligand to the receptor.Many receptor-mediated endocytotic systems have been characterized,including those resulting in internalization of galactose, mannose,mannose 6-phosphate, transferrin, asialoglycoprotein, folate,transcobalamin (vitamin B₁₂), α-2 macroglobulins, insulin, and otherpeptide growth factors such as epidermal growth factor (EGF).

Receptor mediated endocytosis has been utilized for delivering exogenousmolecules such as proteins and nucleic acids to cells. Generally, aspecific ligand is chemically conjugated by covalent, ionic, or hydrogenbonding to an exogenous molecule of interest, forming a conjugatemolecule having a moiety (the ligand portion) that is still recognizedin the conjugate by a target receptor. Using this technique thephototoxic protein psoralen has been conjugated to insulin andinternalized by the insulin receptor endocytotic pathway (Gasparro,Biochem. Biophys. Res. Comm. 141(2), pp. 502-509, Dec. 15, 1986); thehepatocyte specific receptor for galactose terminal asialoglycoproteinshas been utilized for the hepatocyte-specific transmembrane delivery ofasialoorosomucoid-poly-L-lysine non-covalently complexed to a plasmid(Wu, G. Y., J. Biol. Chem., 262(10), pp. 4429-4432, 1987); the cellreceptor for EGF has been utilized to deliver polynucleotides covalentlylinked to EGF to the cell interior (Myers, European Patent Application86810614.7, published Jun. 6, 1988); the intestinally situated cellularreceptor for the organometallic vitamin B₁₂-intrinsic factor complex hasbeen used to mediate delivery of a drug, a hormone, a bioactive peptideand an immunogen complexed with vitamin B₁₂ to the circulatory systemafter oral administration (Russell-Jones et al., European patentApplication 86307849.9, published Apr. 29, 1987); themannose-6-phosphate receptor has been used to deliver low densitylipoproteins to cells (Murray, G. J. and Neville, D. M., Jr., J. Biol.Chem, Vol. 255 (24), pp. 1194-11948, 1980); the cholera toxin bindingsubunit receptor has been used to deliver insulin to cells lackinginsulin receptors (Roth and Maddox, J. Cell. Phys. Vol. 115, p. 151,1983); and the human chorionic gonadotropin receptor has been employedto deliver a ricin a-chain coupled to HCG to cells with the appropriateHCG receptor (Oeltmann and Heath, J. Biol. Chem, vol. 254, p. 1028(1979)).

In one embodiment the present invention involves the transmembranetransport of a radionuclide-based imaging agent across a membrane havingreceptors for a vitamin, or a vitamin receptor binding derivative oranalog thereof. A cell membrane bearing vitamin receptors, or receptorsfor vitamin derivatives or analogs, is contacted with a vitamin-imagingagent conjugate for a time sufficient to initiate and permittransmembrane transport of the conjugate, and the biodistribution of thevitamin-imaging agent conjugate in the animal is monitored. In anotherembodiment, the vitamin/vitamin derivative or analog targeting moietysimply binds to a cell surface vitamin receptor to concentrate thechelated radionuclide on the cell surface.

The invention takes advantage of (1) the location of vitamin receptorsand (2) the associated receptor-mediated endocytic processes. Forexample, the invention takes advantage of the unique expression,overexpression, or preferential expression of vitamin receptors,transporters, or other surface-presented proteins that specifically bindvitamins, or derivatives or analogs thereof, on tumor cells or othercell types which overexpress such receptors. Accordingly, the inventioncan be used to detect cells, such as tumor cells or other cell types,which overexpress vitamin receptors, or receptors for vitaminderivatives or analogs, by taking advantage of the receptor-mediatedendocytic processes that occur when such cells are contacted with thevitamin-imaging agent conjugate.

Vitamin receptors, such as the high-affinity folate receptor (FR) isexpressed at high levels, for example, on cancer cells. Epithelialcancers of the ovary, mammary gland, colon, lung, nose, throat, andbrain have all been reported to express elevated levels of the FR. Infact, greater than 90% of all human ovarian tumors are known to expresslarge amounts of this receptor. Thus, the present invention can be usedfor the diagnostic imaging of a variety of tumor types, and of othercell types involved in disease states.

Radionuclide chelators complexed to ligands have been used asnon-invasive probes for diagnostic imaging purposes. For example,vasoactive intestinal peptide, somatostatin analogs, and monoclonalantibodies have been used as ligands to localize radionuclides to cells,such as tumor cells. Monoclonal antibodies, and various fragmentsthereof, initially received the most attention because it was believedthat precise tumor-specific targeting might be achieved using monoclonalantibodies as targeting ligands. Unfortunately, this approach wasproblematic because i) antibodies have prolonged circulation times dueto their large size which is unfavorable for imaging purposes, ii)antibodies are expensive to produce, iii) antibodies can be immunogenic,and, accordingly, must be humanized when multiple doses are used, andiv) tumor to non-target tissue ratios (T/NT) of antibody-linkedradionuclides are sub-optimal. Thus, the focus has recently beendirected to the use of smaller tumor-specific ligands that do not havesuch limitations.

Vitamins, such as folic acid, have been used for the targeting ofimaging agents to tumor cells, and are advantageous because of theirsmall size. The first folic acid-based targeting complex described forin vivo tumor imaging was a histamine derivative containing ¹²⁵Iodine.This complex was not considered a relevant clinical candidate because ofthe long-lived ¹²⁵I radionuclide component. Subsequently, adeferoxamine-folate conjugate for tumor imaging was developed(deferoxamine chelates ⁶⁷Ga, a gamma-emitting radionuclide that has a 78hour half-life). Hepatobiliary clearance was noted with this conjugateand, thus, preclinical development was stopped due to anticipatedproblems in accurately imaging regio-abdominal locations. This obstaclewas overcome, however, by replacing the deferoxamine chelator withdiethylenetriamine pentaacetic acid (DTPA), an efficient chelator of¹¹¹In (68 hour half life). The primary route of elimination of¹¹¹In-DTPA-folate was confirmed to be through the kidneys.

More recently, ^(99m)Tc has been adopted as the preferred radionuclidefor diagnostic imaging, because i) the radionuclide is easily obtainedfrom commercially available ⁹⁹Mo-^(99m)Tc generators, ii) the cost ofproducing large amounts of ^(99m)Tc is insignificant compared to thecost of producing ¹¹¹In, and iii) ^(99m)Tc has a much shorter (6 hour)half life, which allows higher radionuclide doses to be administered,yielding higher resolution images without the risk of hazardousradiation exposure to vital organs.

Several folate-based ^(99m)Tc conjugates have been developed. Forexample, folate conjugates of ^(99m)Tc-6-hydrazinonicotinamido-hydrazido(HYNIC; Guo, et al., J. Nucl. Med., 40(9): 1563-1569 (1999)),^(99m)Tc˜12-amino-3,3,9,9-tetramethyl-5-oxa-4,8 diaza-2,10-dodecanedinoedioxime (OXA) (Linder, et al., Soc. Nucl. Med., Proc. 47^(th) AnnualMeeting, 2000, 41(5): 119P), ^(99m)Tc-ethylenedicysteine (Ilgan, et al.,Cancer Biother. & Radiopharm., 13(6): 427-435 (1998)), and^(99m)Tc-DTPA˜folate (Mathias, et al., Bioconjug. Chem., 11(2): 253-257(2000)) have shown promising in vivo tumor uptake qualities. However,there is a need for alternative vitamin-based ^(99m)Tc conjugates, orvitamin-based conjugates employing other radionuclides, that exhibitoptimal tumor to non-target tissue ratios (T/NT) and are eliminatedthrough the kidneys. Such vitamin-based conjugates should be suitablefor clinical development as tumor imaging agents, and for the diagnosisof other disease states.

In one embodiment is provided a compound of the formula

wherein V is a vitamin, or a vitamin receptor binding derivative oranalog thereof, L is a divalent linker, R is a side chain of an aminoacid of the formula H₂NCHRCOOH, M is a cation of a radionuclide, n is 1or 0, and k is 1 or 0. The vitamin is a substrate for receptor-mediatedtransmembrane transport in vivo.

In another embodiment is provided a composition for diagnostic imagingcomprising a compound of the formula

wherein V is a vitamin, or a vitamin receptor binding derivative oranalog thereof, L is a divalent linker, R is a side chain of an aminoacid of the formula H₂NCHRCOOH, M is a cation of a radionuclide, n is 1or 0, and a pharmaceutically acceptable carrier therefor. The vitamin isa substrate for receptor-mediated transmembrane transport in vivo.

In yet another embodiment a method is provided of imaging a populationof cells in an animal, wherein the cells are characterized by a vitaminreceptor on the surface of the cells. The method comprises the steps ofadministering to the animal an effective amount of a compositioncomprising a compound of the formula

wherein V is a vitamin, or a receptor binding derivative or analogthereof, specific for the cell surface vitamin receptor, L is a divalentlinker, R is a side chain of an amino acid of the formula H₂NCHRCOOH, Mis a cation of a radionuclide, n is 1 or 0, and a pharmaceuticallyacceptable carrier therefor, and monitoring the biodistribution of thecompound in the animal.

In another embodiment a compound is provided of the formula

wherein V is a vitamin that is a substrate for receptor-mediatedtransmembrane transport in vivo, or a vitamin receptor bindingderivative or analog thereof, L is a divalent linker, R is a side chainof an amino acid of the formula H₂NCHRCOOH, M is a cation of aradionuclide, n is 1 or 0, and k is 1 or 0.

In still another embodiment, a composition for diagnostic imaging isprovided comprising a compound of the formula

wherein V is a vitamin that is a substrate for receptor-mediatedtransmembrane transport in vivo, or a vitamin receptor bindingderivative or analog thereof, L is a divalent linker, R is a side chainof an amino acid of the formula H₂NCHRCOOH, M is a cation of aradionuclide, n is 1 or 0, and a pharmaceutically acceptable carriertherefor.

In yet another embodiment, a method of imaging a population of cells inan animal is provided wherein the cells are characterized by a vitaminreceptor on the surface of the cells. The method comprises the steps ofadministering to the animal an effective amount of a compositioncomprising a compound of the formula

wherein V is the vitamin, or a receptor binding derivative or analogthereof, specific for the cell surface vitamin receptor, L is a divalentlinker, R is a side chain of an amino acid of the formula H₂NCHRCOOH, Mis a cation of a radionuclide, n is 1 or 0, and a pharmaceuticallyacceptable carrier therefor, and monitoring the biodistribution of thecompound in the animal.

In any of these embodiments, V in the compound can be, for example, avitamin selected from the group consisting of folate, riboflavin,thiamine, vitamin B₁₂, and biotin, or a derivative or analog thereof. Inany of these embodiments, the radionuclide in the compound can beselected, for example, from the group consisting of radioisotopes ofgallium, indium, copper, technetium, and rhenium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Structure of EC20, an exemplary compound used as an imagingagent in accordance with the invention.

FIG. 2. HPLC radiochromatogram of ^(99m)Tc-EC20. Samples of^(99m)Tc-EC20 were eluted isocratically on a Waters Nova-Pak C18(3.9×150 mm) column using an aqueous mobile phase containing 20%methanol and 0.2% trifluoroacetic acid at a flow rate of 1 mL/min. TheHPLC analysis was monitored with both the UV detector (280 nm) and aBioscan FC-3200 radiodetector. Peak A, free ^(99m)Tc; Peak B, afolate-containing chelate of unknown structure; Peaks C and D,diastereomers possessing either a syn or anti configuration of thetechnetium-oxygen bond in the Dap-Asp-Cys chelating ring of EC20.

FIG. 3. Structures of Re-EC20 and ^(99m)Tc-EC20 isomers (syn or antiposition of metal-oxo bond).

FIG. 4. Blocking of ³H-folic acid binding to KB cells with variousfolate-containing competitors. KB cells were incubated for 15 min on icewith 100 nM ³H-folic acid in the presence and absence of increasingcompetitor concentrations. (•) Folic acid; (▪) EC20; (▴) EC20:Re isomerA; (▾) EC20:Re isomer B; (□) DTPA-Folate. Error bars represent 1standard deviation (n=3).

FIG. 5. Time-dependent association of ^(99m)Tc-EC20. KB cells wereincubated with 10 nM ^(99m)Tc-EC20 for increasing periods of time at 37°C. Following multiple washes, cells were harvested and counted forassociated radioactivity. Error bars represent 1 standard deviation(n=3).

FIG. 6. Concentration-dependent association of ^(99m)Tc-EC20. KB cellswere incubated for 2 hr at 37° C. in the presence of increasingconcentrations of ^(99m)Tc-EC20. Following multiple washes, cells wereharvested and counted for associated radioactivity. Error bars represent1 standard deviation (n=3).

FIG. 7. Concentration-dependent association of ^(99m)Tc-EC20 “peak B.”KB cells were incubated for 2 hr at 37° C. in the presence of increasingconcentrations of “Peak B” that was chromatographically isolated fromthe ^(99m)Tc-EC20 formulation. Following multiple washes, cells wereharvested and counted for associated radioactivity. Error bars represent1 standard deviation (n=3). (•), Peak B; (∘), Peak B plus 1 mM folicacid.

FIG. 8. Blood clearance of ^(99m)Tc-EC20 in Balb/c mice. Each animalreceived an intravenous dose of 50 μg/kg EC20 (67 nmol/kg) inapproximately 0.1 mL during brief diethyl ether anesthesia. At thedesignated times post-injection, each animal was euthanized by CO₂asphyxiation, blood was collected and counted for associatedradioactivity. Error bars represent 1 standard deviation (n=3 animals).

FIG. 9. Whole-body gamma images (ventral view). Images were obtained 4hr following intravenous administration of ^(99m)Tc-EC20 to a Balb/cmouse bearing a subcutaneous folate receptor-positive M109 tumor. Onlythe kidneys (K) and tumor (T) exhibit significant accumulation of thisradiotracer.

FIG. 10. Structures of EC11, EC13, EC14, EC15, EC19, EC20, EC31, andEC53.

FIG. 11. Tissue distribution of ^(99m)Tc-EC20 in Balb/c mice bearingFR-postive M109 tumors and FR-negative 4T1 tumors.

FIG. 12. HPLC analysis of EC11.

FIG. 13. Mass spectroscopy analysis of EC11.

FIG. 14. NMR analysis of EC11.

FIG. 15. HPLC analysis of EC13.

FIG. 16. NMR analysis of EC14.

FIG. 17. Mass spectroscopy analysis of EC15.

FIG. 18. HPLC analysis of EC19.

FIG. 19. Mass spectroscopy analysis of EC19.

FIG. 20. HPLC analysis of EC31.

FIG. 21. HPLC analysis of EC53.

FIG. 22. Mass spectroscopy analysis of EC53.

FIG. 23. Mass spectroscopy analysis of EC53.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the invention, compounds and methods are provided fortargeting radionuclide-based imaging agents to cell populations thatuniquely express, overexpress, or preferentially express vitaminreceptors. Accordingly, a vitamin, or a receptor binding derivative oranalog thereof, is used as the targeting ligand for the imaging agent.The vitamin-imaging agent conjugate can be used to target radionuclidesto cells and to concentrate the radionuclides in a cell population, suchas a tumor cell population, for use in diagnostic imaging.

The invention provides a composition for diagnostic imaging comprising acompound of the formula

for use in such methods. In the compound, V is a vitamin, or a vitaminreceptor binding derivative or analog thereof, L is a divalent linker, Ris a side chain of an amino acid of the formula H₂NCHRCOOH, M is acation of a radionuclide, and n is 1 or 0. The vitamin, or vitaminreceptor binding derivative or analog thereof, is a substrate forreceptor-mediated transmembrane transport in vivo.

The invention also provides compounds of the formulas

wherein V is a vitamin, or a vitamin receptor binding derivative oranalog thereof, L is a divalent linker, R is a side chain of an aminoacid of the formula H₂NCHRCOOH, M is a cation of a radionuclide, n is 1or 0, and k is 1 or 0. The vitamin is a substrate for receptor-mediatedtransmembrane transport in vivo.

Exemplary of these compounds is a compound referred to as EC20 depictedin FIG. 1. Exemplary of other compounds for use in accordance with thisinvention are compounds denominated as EC11, EC13, EC14, EC15, EC19,EC31, and EC53 (see FIG. 10). The vitamin moiety (e.g., the folic acidmoiety in EC20) provides high affinity binding to cellular FRs. Thecompounds also contain a bifunctional peptide-based chelator, whichprovides the site for chelation of the radionuclide, for example,^(99m)Tc (see FIG. 1), and the compounds can, optionally, contain alinker through which the vitamin moiety is covalently bonded to thechelating moiety.

In accordance with the invention, the vitamin moiety of the compounds isa vitamin that is a substrate for receptor-mediated transmembranetransport in vivo, or a vitamin receptor binding derivative or analogthereof The vitamin is linked, optionally, through a linker (L) to thechelator portion of the compounds. The chelator portion comprises an α,β-diaminopropionic acid moiety linked to a cysteine group through athird amino acid residue. The chelator portion of the compound isadapted to bind a radionuclide cation (M) (where k=1).

In accordance with the invention, the compounds with bound radionuclideare referred to as “vitamin-imaging agent conjugates.”

The structure of the linker, if present, is not critical to theinvention. Thus, for example, it can be any biocompatible divalentlinker. Typically, the linker comprises about 1 to about 30 carbonatoms, more typically about 2 to about 20 carbon atoms. Lower molecularweight linkers (i.e., those having an approximate molecular weight ofabout 30 to about 300) are typically employed. Furthermore, the vitaminmoiety may be a vitamin, or a derivative or analog thereof. For example,folate contains one glutamic acid in the L configuration linked topteroic acid. As shown in FIG. 1, EC20 comprises a folic acid analoglinked to the chelator moiety because EC20 has the glutamic acid in theD configuration. EC11 and EC14 contain two glutamic acid residues and,thus, these compounds can also, for example, be considered derivativesof folic acid (FIG. 10).

Among vitamins believed to trigger receptor-mediated endocytosis andhaving application in accordance with the presently disclosed method areniacin, pantothenic acid, folic acid, riboflavin, thiamine, biotin,vitamin B₁₂, and the lipid soluble vitamins A, D, E and K. Thesevitamins, and their analogs and derivatives, constitute vitamins thatcan be coupled with imaging agents to form the vitamin-chelatorconjugates for use in accordance with the invention. Preferred vitaminmoieties include folic acid, biotin, riboflavin, thiamine, vitamin B₁₂,and analogs and derivatives of these vitamin molecules, and otherrelated vitamin receptor-binding molecules.

Folic acid, folinic acid, pteroic acid, pteropolyglutamic acid, andfolate receptor-binding pteridines such as tetrahydropterins,dihydrofolates, tetrahydrofolates, and their deaza and dideaza analogscan be used in accordance with the invention. The terms “deaza” and“dideaza” analogs refers to the art-recognized folate analogs having acarbon atom substituted for one or two nitrogen atoms in the naturallyoccurring folic acid structure. For example, the deaza analogs includethe 1-deaza, 3-deaza, 5-deaza, 8-deaza, and 10-deaza analogs. Thedideaza analogs include, for example, 1,5 dideaza, 5,10-dideaza,8,10-dideaza, and 5,8-dideaza analogs. The foregoing are folate analogsor derivatives and can bind to folate receptors. Other folate analogs orderivatives useful in accordance with the invention are the folatereceptor-binding analogs aminopterin, amethopterin (methotrexate),N¹⁰-methylfolate, 2-deamino-hydroxyfolate, deaza analogs such as1-deazamethopterin or 3-deazamethopterin, and3′5′-dichloro-4-amino-4-deoxy-N¹⁰-methylpteroylglutamic acid(dichloromethotrexate).

The vitamin, or derivative or analog thereof, can be capable ofselectively binding to the population of cells to be visualized due topreferential expression on the targeted cells of a receptor for thevitamin, or derivative or analog, wherein the receptor is accessible forbinding. The binding site for the vitamin can include receptors for anyvitamin molecule capable of specifically binding to a receptor whereinthe receptor or other protein is uniquely expressed, overexpressed, orpreferentially expressed by the population of cells to be visualized. Asurface-presented protein uniquely expressed, overexpressed, orpreferentially expressed by the cells to be visualized is a receptor notpresent or present at lower amounts on other cells providing a means forselective, rapid, and sensitive visualization of the cells targeted fordiagnostic imaging using the vitamin-imaging agent conjugates of thepresent invention.

In accordance with the invention the vitamin-imaging agent conjugatesare capable of high affinity binding to receptors on cancer cells orother cells to be visualized. The high affinity binding can be inherentto the vitamin moiety or the binding affinity can be enhanced by the useof a chemically modified vitamin (i.e., an analog or a derivative) or bythe particular chemical linkage between the vitamin and the chelatormoiety that is present in the conjugate.

In accordance with the invention, the chelator can be conjugated withmultiple, different vitamins, or vitamin receptor binding derivatives oranalogs, to enhance the opportunity for binding to the respective cellmembrane receptors. Alternatively, independent portions of the dose of avitamin-imaging agent conjugate can constitute different vitamin-imagingagent conjugates to enhance the opportunity for binding to therespective cell membrane receptors.

Generally, any manner of forming a complex between the chelator and thevitamin, or vitamin receptor binding derivative or analog, can beutilized in accordance with the present invention. The chelator can forma complex with the vitamin, or vitamin receptor binding derivative oranalog, by direct conjugation of the chelator and the vitamin by using adivalent linker. Alternatively, the vitamin and the chelator may beconjugated without employing a linker. If a linker is used, the linkercan directly conjugate the vitamin, or vitamin receptor bindingderivative or analog, and the chelator through a hydrogen, ionic, orcovalent bond. Also, in accordance with this invention the divalentlinker can comprise an indirect means for associating the chelator withthe vitamin, or vitamin receptor binding derivative or analog, such asby connection through intermediary linkers, spacer arms, or bridgingmolecules. Both direct and indirect means for association must notprevent the binding of the vitamin, or vitamin receptor bindingderivative or analog, to the vitamin receptor on the cell membrane foroperation of the method of the present invention.

Covalent bonding of the vitamin, or vitamin receptor binding derivativeor analog, and the chelator can occur, whether or not a linker isemployed, through the formation of amide, ester or imino bonds betweenacid, aldehyde, hydroxy, amino, or hydrazo groups. For example, acarboxylic acid on the vitamin moiety or on the chelator can beactivated using carbonyldiimidazole or standard carbodiimide couplingreagents such as 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC)and thereafter reacted with the other component of the conjugate, orwith a linker, having at least one nucleophilic group, viz hydroxy,amino, hydrazo, or thiol, to form the vitamin-chelator conjugatecoupled, with or without a linker, through ester, amide, or thioesterbonds.

The radionuclides suitable for diagnostic imaging include radioisotopesof gallium, indium, copper, technetium and rhenium, including isotopes¹¹¹In, ^(99m)Tc, ⁶⁴Cu, ⁶⁷Cu, ⁶⁷Ga or ⁶⁸Ga. These radionuclides arecationic and are complexed with the chelator through the chelating groupof the conjugate to form the vitamin-imaging agent conjugate.

The vitamin-imaging agent conjugates in accordance with the inventionare utilized to selectively visualize, using scintigraphic imagingtechniques, a population of cells in an animal wherein the population ofcells uniquely expresses, overexpresses, or preferentially expressesreceptors for a vitamin, or a vitamin receptor binding derivative oranalog thereof. The vitamin-imaging agent conjugates can be used tovisualize populations of pathogenic cells, as long as the cells uniquelyor preferentially express or overexpress vitamin receptors or receptorsthat bind vitamin derivatives or analogs.

The invention is applicable to populations of pathogenic cells thatcause a variety of pathologies including cancer, and diseases mediatedby any other type of pathogenic cells that overexpress vitaminreceptors, or receptors capable of binding vitamin derivatives oranalogs. Thus, the population of pathogenic cells can be tumorigenic,including benign tumors and malignant tumors, or it can benon-tumorigenic. If the cell population is a cancer cell population, thecancer cells can arise spontaneously or by such processes as mutationspresent in the germline of the host animal or somatic mutations, or thecancer can be chemically-, virally-, or radiation-induced. The inventioncan be utilized for diagnostic imaging of such cancers as carcinomas,sarcomas, lymphomas, Hodgekin's disease, melanomas, mesotheliomas,Burkitt's lymphoma, nasopharyngeal carcinomas, and myelomas. The cancercell population can include, but is not limited to, oral,nasopharyngeal, thyroid, endocrine, skin, gastric, esophageal,laryngeal, throat, pancreatic, colon, bladder, bone, ovarian, cervical,uterine, breast, testicular, prostate, rectal, kidney, liver, lung, andbrain cancers. In embodiments where the cell population is a cancer cellpopulation, tumor cells, including cells of the primary tumor or cellsthat have metastasized or are in the process of dissociating from theprimary tumor, can be visualized using the vitamin-imaging agentconjugate.

The vitamin-imaging agent conjugates of the present invention can beused to diagnose a disease state or to monitor the progression ofdisease. For example, the diagnostic imaging method in accordance withthe invention can be used to monitor the progression of cancer incombination with prophylactic treatments to prevent return of a tumorafter its removal by any therapeutic approach including surgical removalof the tumor, radiation therapy, chemotherapy, or biological therapy.

The compositions and methods of the present invention can be used forboth human clinical medicine and veterinary applications. Thus, theanimal harboring the population of cells that are visualized can behuman or, in the case of veterinary applications, can be a laboratory,agricultural, domestic, or wild animal. The present invention can beapplied to animals including, but not limited to, humans, laboratoryanimals such rodents (e.g., mice, rats, hamsters, etc.), rabbits,monkeys, chimpanzees, domestic animals such as dogs, cats, and rabbits,agricultural animals such as cows, horses, pigs, sheep, goats, and wildanimals in captivity such as bears, pandas, lions, tigers, leopards,elephants, zebras, giraffes, gorillas, dolphins, and whales.

The compositions for diagnostic imaging comprise an amount of thevitamin-imaging agent conjugate effective to visualize the cellstargeted for diagnostic imaging in an animal when administered in one ormore doses. The diagnostic imaging composition containing thevitamin-imaging agent conjugate is preferably administered to the animalparenterally, e.g., intradermally, subcutaneously, intramuscularly,intraperitoneally, intravenously, or intrathecally. Alternatively, thecomposition containing the vitamin-imaging agent conjugate can beadministered to the animal by other medically useful processes, and anyeffective dose and suitable dosage form can be used, including oral andinhalation dosage forms.

Examples of parenteral dosage forms include aqueous solutions of thevitamin-imaging agent conjugate, in isotonic saline, 5% glucose or otherwell-known pharmaceutically acceptable liquid carriers such as liquidalcohols, glycols, esters, and amides. The parenteral dosage form inaccordance with this invention can be in the form of a reconstitutablelyophilizate comprising the dose of the vitamin-imaging agent conjugate.

The dosage of the vitamin-imaging agent conjugate in the diagnosticimaging composition can vary significantly depending on the size of theanimal, the cell population targeted for diagnostic imaging, thespecific vitamin-imaging agent conjugate being used, and the route ofadministration of the conjugate. The effective amount to be administeredto the animal is based on body surface area, weight, and physicianassessment of the condition of the animal. An effective dose can rangefrom about 1 ng/kg to about 1 mg/kg, more preferably from about 100ng/kg to about 500 μg/kg, and most preferably from about 100 ng/kg toabout 25 μg/kg.

Any effective regimen for administering the diagnostic imagingcomposition containing the vitamin-imaging agent conjugate can be used.For example, the diagnostic imaging composition can be administered as asingle dose, or it can be administered in multiple doses, if necessary,to achieve visualization of the targeted cell population. Additionalinjections of the diagnostic imaging composition containing thevitamin-imaging agent conjugate can be administered to the animal at aninterval of days or months after the initial injections(s), and theadditional injections can be useful for monitoring the progress of thedisease state. The diagnostic imaging composition containing thevitamin-imaging agent conjugate can also be administered in combinationwith unlabeled vitamin. “In combination with” means that the unlabeledvitamin can be either coadministered with the imaging agent or theunlabeled vitamin can be preinjected before administration of theimaging agent to improve image quality. For example, the imaging agentcan be administered in combination with about 0.5 ng/kg to about 100mg/kg, or about 1 μg/kg to about 100 mg/kg, or about 100 μg/kg to about100 mg/kg of the unlabeled vitamin.

The diagnostic imaging composition is typically formulated forparenteral administration and is administered to the animal in an amounteffective to enable imaging of the targeted cell population. Typically,the diagnostic imaging composition containing the vitamin-targetedimaging agent is administered to the animal, and following a period oftime to allow delivery and concentration of the vitamin-imaging agentconjugate in the targeted cell population, the animal is subjected tothe imaging procedure and imaging is enabled by the vitamin-imagingagent conjugate. When used for monitoring the progression of disease ordiagnosis, imaging procedures are typically carried out about 1 to about6 hours post administration of the diagnostic imaging compositioncontaining the vitamin-imaging agent conjugate.

The invention also provides a method of imaging a population of cells inan animal wherein the cells are characterized by a vitamin receptor onthe surface of the cells. The method comprises the steps ofadministering to the animal an effective amount of a compositioncomprising a compound of the formula

wherein V is the vitamin, or a receptor binding derivative or analogthereof, specific for the cell surface vitamin receptor, L is a divalentlinker, R is a side chain of an amino acid of the formula H₂NCHRCOOH, Mis a cation of a radionuclide, n is 1 or 0, and a pharmaceuticallyacceptable carrier therefor, and monitoring the biodistribution of thecompound in the animal.

The method can be used to image a cell population in vitro, e.g., incell culture, or in vivo, where the cells form part of or otherwiseexist in animal tissue. Thus, the target cells can include, for example,the cells lining the alimentary canal, such as the oral and pharyngealmucosa, the cells forming the villi of the small intestine, or the cellslining the large intestine. Such cells of the alimentary canal can betargeted in accordance with this invention by oral administration of adiagnostic imaging composition comprising the vitamin-imaging agentconjugate. Similarly, cells lining the respiratory system (nasalpassages/lungs) of an animal can be targeted by inhalation of thepresent complexes, and cells of internal organs, including cells of theovaries and the brain can be targeted, particularly, by parenteraladministration of the diagnostic imaging composition.

EXAMPLE 1

Materials.

N¹⁰-trifluoroacetylpteroic acid was purchased from Eprova AG,Schaffhausen, Switzerland. Peptide synthesis reagents were purchasedfrom NovaBiochem and Bachem. ^(99m)Tc Sodium Pertechnetate was suppliedby Syncor. [ReO₂(en)₂]C1 was prepared according to Rouschias (Rouschias,G., Chem. Rev., 74: 531 (1974)). Cellulose plates and DEAE ion exchangeplates were purchased from J. T. Baker.

EXAMPLE 2

Synthesis, Purification, and Analytical Characterization of EC20.

EC20 was prepared by a polymer-supported sequential approach using theFmoc-strategy (see Scheme 1 below; Fmoc=9-fluorenylmethyloxycarbonyl;Boc=tert.butyloxycarbonyl; Dap=diaminopropionic acid;DMF=dimethylformamide; DIPEA=diisopropylethylamine). EC20 wassynthesized on an acid-sensitive Wang resin loaded withFmoc-_(L)-Cys(Trt)-OH.Benzotriazole-1-yl-oxy-tris-pyrrolidino-phosphoniumhexafluorophosphate(PyBOP) was applied as the activating reagent to ensure efficientcoupling using low equivalents of amino acids. Fmoc protecting groupswere removed after every coupling step under standard conditions (20%piperidine in DMF). After the last assembly step the peptide was cleavedfrom the polymeric support by treatment with 92.5% trifluoroacetic acidcontaining 2.5% ethanedithiol, 2.5% triisopropylsilane and 2.5%deionized water. This reaction also resulted in simultaneous removal ofthe t-Bu, Boc and trityl protecting groups. Finally, the trifluoroacetylmoiety was removed in aqueous ammonium hydroxide to give EC20.

The crude EC20 product was purified by HPLC using an Xterra RP18 30×300mm, 7 μm column (Waters); mobile phase 32 mM HC1 (A), MeOH (B); gradientconditions starting with 99% A and 1% B, and reaching 89% A and 11% B in37 min by a flow rate of 20 mL/min. Under these conditions, EC20 monomertypically eluted at 14.38 min, whereas EC20 disulfide dimer (minorcontaminant) eluted at 16.83 min. All other compounds shown in FIG. 10can be prepared using a similar synthesis scheme except for EC15 whichis synthesized as shown in Scheme 2 below.

Two milligrams of HPLC-purified EC20 were dissolved in 0.62 mL of D₂O,and a 500 MHz ¹H-NMR spectrum was collected. Table 1 (see below) liststhe chemical shifts, signal shapes, and J values for allnon-exchangeable protons in EC20 molecule.

EC20 was also analyzed by electrospray-mass spectrometry. Major positiveion peaks (m/z, relative intensity): 746.1, 100; 747.1, 44; 556.8, 32;570.8, 16.

TABLE 1 ¹H-NMR data for EC20. EC20 was dissolved in D₂O and a 500 MHzspectrum was collected. Chemical shifts (δ) are in ppm. The signal forHOD at δ = 4.80 ppm was used as the reference. Protons Chemical Residueobserved Shift (δ) Signals J values Pte H-7 8.76 s ³J12, 13=³J15, 2 ×H-9 4.64 s 16=8.8Hz H-12 a. H-16 7.68 d H-13 a. H-15 6.8 d D-Glu H-24.41 dd ³J2, 3=³9.1Hz; H-3a 2.08 m ³J2, 3b=4.5Hz H-3B 2.27 m ²J3a,3b=14.2Hz 2 × H-4 2.44 dd ³Ja, 4=³J4b, 4=5.6Hz Dpr H-2 4.1 dd; X of ABX³J2, 3A=6.6Hz; H-3A 3.52 System ³J2, 3B=4.7Hz H-3B 3.72 dd; A of ABX²JA, B=14.7Hz System dd; B of ABX System Asp H-2 4.71 dd; X of ABX ³J2,3A=9.5hz; H-3A 2.62 System ³J2, 3B=4.3Hz H-3B 2.81 dd; A of ABX ²JA,B=16.1Hz System dd; B of ABX System Cys H-2 4.3 dd; X of ABX ³J2,3A=5.5hz; H-3A 2.85 System ³J2, 3B=4.4Hz H-3B 2.89 dd; A of ABX ²JA,B=14.1Hz System dd; B of ABX SystempD = 4.78;s = singlet;d = doublet;m = multiplet.

EXAMPLE 3

Preparation of the Non-Radioactive Reagent vial and of ^(99m)Tc-EC20.

EC20 kits were used for preparation of the ^(99m)Tc-EC20 radioactivedrug substance. Each kit contained a sterile, non-pyrogenic lyophilizedmixture of 0.1 mg EC20, 80 mg sodium α-_(D)-glucoheptonate, 80 mg tin(II) chloride dihydrate, and sufficient sodium hydroxide or hydrochloricacid to adjust the pH to 6.8±0.2 prior to lyophilization. Thelyophilized powder was sealed in a 5 mL vial under an argon atmosphere.The kits were then stored frozen at −20° C. until use or expiration(current shelf life is >2 years). Importantly, the tin (II) chloridecomponent is required to reduce the added ^(99m)Tc-pertechnetate, whilethe sodium α-_(D)-glucoheptonate component is necessary to stabilize thenewly reduced ^(99m)Tc prior to its final chelation to the EC20compound.

^(99m)Tc-EC20 was prepared as follows (i.e., chelation of ^(99m)Tc toEC20). First, a boiling water bath containing a partially submerged leadvial shield was prepared. The top of an EC20 vial was swabbed with 70%ethanol to sanitize the surface and the vial was placed in a suitableshielding container. Using a shielded syringe with 27-gauge needle, 1 mLof sterile Sodium Pertechnetate ^(99m)Tc Injection (15 to 20 mCi) in0.9% sodium chloride was injected into the shielded vial. Before removalof the syringe from the vial, a volume of gas from the vial equal to thevolume of pertechnetate added was withdrawn in order to normalize thepressure inside the vial. The vial was gently swirled for 30 seconds toensure complete dissolution of the lyophilized powder. The vial was thenplaced into the lead shield that was standing in the boiling water bath.The solution was heated for ˜18 minutes and then cooled to roomtemperature for a minimum of 15 min. This solution can be stored at roomtemperature (15-25° C.) protected from light, but it should be usedwithin 6 hours of preparation.

The radiochemical stability of the radioactive drug substance wasdetermined by HPLC after storing at room temperature protected fromlight for up to 24 hours. Samples of the ^(99m)Tc-EC20 solution (20 μL)were analyzed using an HPLC system consisting of a Waters 600EMultisolvent Delivery System and 490 UV detector, a Bioscan EC-3200radiodetector, Laura v1.5 radiochromatogram software, and a WatersNova-Pak C18 (3.9×150 mm) column. Injected samples were elutedisocratically using an aqueous mobile phase containing 20% methanol and0.1% trifluoroacetic acid at a flow rate of 1 mL/min. The HPLC analysiswas monitored with both the UV detector (280 nm) and the gammaradiodetector. Notably, the radiochemical purity of ^(99m)Tc-EC20remained greater than 90% for at least 24 hours in all cases.

EXAMPLE 4

Determination of Radiochemical purity of ^(99m)Tc-EC20 by TLC.

The major radiochemical impurities in the preparation of ^(99m)Tc-EC20will be 1) ^(99m)Tc pertechnetate, 2) ^(99m)Tc-glucoheptonate (ligandexchange precursor), 3) non-specific binding ^(99m)Tc (^(99m)Tc bound ata site other than the expected Dap-Asp-Cys chelating moiety of the EC20molecule), and 4) hydrolyzed ^(99m)Tc. Since ^(99m)Tc-EC20 was beingtested for possible clinical use, a three-TLC-based method was developedto determine the amounts of each impurity and to estimate the overallradiochemical purity of ^(99m)Tc-EC20.

In the first system a cellulose plate was developed with deionizedwater. ^(99m)Tc-EC20, ^(99m)Tc-glucoheptonate, non-specific binding^(99m)Tc and ^(99m)Tc pertechnetate move to the solvent front(R_(f)=1.0), while hydrolyzed ^(99m)Tc remains at the origin(R_(f)=0.0). The cellulose plate was cut into two pieces at R_(f)=0.3(1.5 cm from origin) and each piece was counted using a dose calibrator.The percent of hydrolyzed ^(99m)Tc was calculated as follows: A=%Hydrolyzed; ^(99m)Tc=(μCi in bottom piece/μCi in both pieces)×100.

In the second system, a cellulose plate was developed with acetone and0.9% NaCl (7:3,v/v). ^(99m)Tc-pertechnetate moves with R_(f)=0.9, while^(99m)Tc-EC20, ^(99m)Tc-glucoheptonate, non-specific binding ^(99m)Tcand hydrolyzed ^(99m)Tc remain at the origin (R_(f)=0.0). Thecellulose/acetone-saline plate was cut into two pieces at R_(f)=0.6 (3.0cm from the origin) and each piece was counted using a dose calibrator.The percent of ^(99m)Tc-pertechnetate was calculated as follows: B=%^(99m)Tc-pertechnetate=(μCi in top piece/μCi in both pieces)×100.

Finally, in the third system a DEAE ion exchange plate was developedwith 0.3 M Na₂SO₄. ^(99m)Tc-glucoheptonate moves to the solvent front(R_(f)=1.0), nonspecific binding ^(99m)Tc moves with R_(f)=0.6, and^(99m)Tc˜EC20, hydrolyzed ^(99m)Tc and ^(99m)Tc-pertechnetate remainnear the origin (^(99m)Tc-EC20. R_(f)=0.1; hydrolyzed ^(99m)Tc:R_(f)=0.0; ^(99m)Tc pertechnetate: R_(f)=0.3). The cellulose/Na₂SO₄plate was cut into two pieces at 2.5 cm from the origin and each piecewas counted using a dose calibrator. The percent of^(99m)Tc-glucoheptonate and non-specific binding ^(99m)Tc werecalculated as follows: C=% (^(99m)Tc-Glucoheptonate+non-specific binding^(99m)Tc)=(μCi in top piece/μCi in both pieces)×100. The overallradiochemical purity of ^(99m)Tc-EC20 was then calculated as follows:Radiochemical purity=100−(A+B+C).

As shown in FIG. 2, HPLC analysis of the ^(99m)Tc-EC20 formulation showsfour radiochemical components, designated as Peaks A through D. Peak Awas confirmed to be free ^(99m)Tc and this by-product is reproduciblypresent at <2%. Peak B, which was different from that of^(99m)Tc-glucoheptonate (data not shown) eluted with a retention time of2.8 min. This component represented about 3% of the mixture and wasthought to result from ^(99m)Tc chelating at some other site on the EC20molecule besides the expected Dap-Asp-Cys moiety. Peaks C and D(retention times of 4.8 minutes and 13.2 minutes, respectively), accountfor the majority of the formulated radiochemical activity.

EXAMPLE 5

Synthesis of Re-EC20.

Fifty-two mg (0.010 mmol) of EC20 and [ReO₂(en)₂]Cl (52 mg, 0.14 mmol)were dissolved in 6 mL and 1 mL argon-purged phosphate buffer (0.05 M,pH 5.8), respectively. The two solutions were combined and heated underan argon atmosphere in a boiling water bath for 2 hours. The reactionmixture was frozen and lyophilized overnight. The crude product waspurified by HPLC (Xterra RPI8 column, 19×150 mm, 10 mM NH₄OAc/CH₃CN,flow rate 10 mL/mm; gradient 1% to 8%). The fractions were collected,lyophilized and stored at −20° C. until use.

Because no mass spectral facilities were available for analysis ofradioactive materials, the non-radioactive rhenium analog, Re-EC20, wasanalyzed. Both rhenium and technetium are Group VIIA metals that havesignificant similarity in physical and chemical properties. They alsoform similar complexes with organic ligands. This analogous chemicalbehavior has been frequently used in structure elucidation of newclasses of technetium radiopharmaceuticals based on non-radioactiverhenium analogues. Interestingly, HPLC analysis of Re-EC20 also showedtwo major peaks eluting at 5 and 14.2 minutes, respectively, similar toPeaks C and D for ^(99m)Tc-EC20 (chromatogram not shown). Mass spectralanalysis confirmed that these two components were isomers correspondingto the Re-EC20 complex (m/z=945). In fact, these species were likelydiastereomers possessing either a syn or anti configuration of thetechnetium-oxygen bond in the Dap-Asp-Cys chelating ring, as depicted inFIG. 3. Because i) the two peaks in the Re-EC20 chromatogram representisomeric complexes, and ii) reports of similar isomerism in technetiumcomplexes exist, it is likely that components C and D in the^(99m)Tc-EC20 radiochromatogram are also isomers.

EXAMPLE 6

Cell Culture.

Cells were grown continuously as a monolayer using folate-free RPMImedium (FFRPMI) containing 10% heat-inactivated fetal calf serum (HIFCS)at 37° C. in a 5% CO₂/95% air-humidified atmosphere with no antibiotics.The HIFCS contained its normal complement of endogenous folates whichenabled the cells to sustain growth in this morephysiologically-relevant medium. All cell experiments were performedusing FFRPMI containing 10% HIFCS (FFRPMI/HIFCS) as the growth medium,except where indicated.

EXAMPLE 7

Relative Affinity Assay.

The relative affinity of various folate derivatives was determinedaccording to the method described by Westerhoff et al. (Mol. Pharm., 48:459-471 (1995)) with slight modification. Briefly, FR-positive KB cellswere gently trypsinized in 0.25% trypsin/PBS at room temperature for 3minutes and then diluted in FFRPMI/HIFCS. Following a 5 min 800×g spinand one PBS wash, the final cell pellet was suspended in FFRPMI 1640 (noserum). Cells were incubated for 15 min on ice with 100 nM of ³H-folicacid in the absence and presence of increasing concentrations offolate-containing test articles. Samples were centrifuged at 10,000×gfor 5 min, cell pellets were suspended in buffer, were transferred toindividual vials containing 5 mL of scintillation cocktail, and werethen counted for radioactivity. Negative control tubes contained onlythe ³H-folic acid in FFRPMI (no competitor). Positive control tubescontained a final concentration of 1 mM folic acid, and CPMs measured inthese samples (representing non-specific binding of label) weresubtracted from all samples. Notably, relative affinities were definedas the inverse molar ratio of compound required to displace 50% of³H-folic acid bound to KB FR, and the relative affinity of folic acidfor the FR was set to 1.

The capacity of EC20 to directly compete with folic acid for binding tocell surface FRs was measured using this assay. Importantly, a relativeaffinity value of 1.0 implies that the test article ligand has anaffinity for the FR equal to folic acid. Likewise, values lower thanunity reflect weaker affinity, and values higher than unity reflectstronger affinity.

Cultured KB cells were incubated with 100 nM ³H-folic acid in thepresence of increasing concentrations of non-radioactive folic acid,EC20, Rhenium-EC20 (isomer A; Peak C), Rhenium-EC20 (isomer B; peak 0),or a related folate-based radiopharmaceutical, DTPA-folate. Following a15-minute incubation at 4° C., cells were rinsed free of unboundmaterial and counted for residual cell-associated radioactivity. Thequantity of bound radioactivity was plotted against the concentration ofunlabeled ligand, and IC₅₀ values (concentration of ligand required toblock 50% of ³H-folic acid binding) were estimated. As shown in FIG. 4and Table 2 (below), EC20 was determined to have an affinity of 0.92relative to that of folic acid for human FRs. Both isomers ofRhenium-EC20 displayed relative affinity values that were very similarto, if not better than, the parent EC20 molecule (1.42 and 1.37 forRe-EC20 isomer A and isomer B, respectively). DTPA-folate, an¹¹¹In-chelating folate radiopharmaceutical agent, displayed a relativeaffinity of 0.87 for the folate receptor. Thus, chemical modification offolate with various metal chelating motifs did not disturb the vitamin'sintrinsic affinity for the FR. TABLE 2 Relative Affinity Estimations.Relative affinities (RA) were defined as the inverse molar ratio ofcompound required to displace 50% of ³H-folic acid bound to FR-positiveKB cells. The relative affinity of folic acid was set to 1. Each testarticle was evaluated in triplicate. Test Article IC₅₀ (nM) S.D. RA S.D.Folic Acid 118 ±19 1.00 EC20 128 ±25 0.92 ±0.23 EC20:Re isomer 1 83 ±161.42 ±0.36 EC20:Re isomer 2 86 ±3 1.37 ±0.23 DTPA-Folate 136 ±12 0.87±0.16

EXAMPLE 8

Time-Dependent Cell Uptake.

KB cells were seeded in 12-well Falcon plates and allowed to formsub-confluent monolayers overnight. Following one rinse with 1 mL offresh FFRPMI/HIFCS, each well received 1 mL of FFRPMI/HIFCS containing10 nM ^(99m)Tc-EC20. Cells were incubated for predetermined times at 37°C. and then rinsed four times with 1 mL of ice-cold PBS, pH 7.4. Thecell monolayers were dissolved in 0.5 mL of PBS, pH 7.4 containing 1%sodium dodecyl sulfate for 15 min at room temperature and then countedfor radioactivity using a Packard gamma counter. The protein in eachsample was quantitated using a BioRad DC Protein Assay kit, and cellularprotein values were converted to cell number using the conversion factorof 2.23×10⁻⁷ mg protein per cell. Final tabulated values were expressedin terms of molecules of EC20 per cell.

The kinetics of ^(99m)Tc-EC20 uptake into FR-positive KB cells wasquantitatively measured using this protocol. As shown in FIG. 5,steady-state uptake was reached within two hours at 37° C., whereapproximately 3.2 million molecules of EC20 were cell-associated,whereas half-maximal cell association occurred 9 minutes after mixing 10nM of this radiopharmaceutical with the cells. Interestingly, thehalf-maximal saturation point was reached in only 37 seconds when cellswere incubated with a 10-fold higher concentration of ^(99m)-Tc-EC20(100 mM; data not shown).

EXAMPLE 9

Concentration-Dependent Cell Uptake.

KB cells were seeded in 12-well Falcon plates and allowed to formsub-confluent monolayers overnight. Following one rinse with 1 mL offresh FERPMI/HIFCS, each well received 1 mL of FFRPMI/HIFCS containingincreasing concentrations of ^(99m)Tc-EC20. Cells were incubated for 2hours at 37° C. and then rinsed four times with 1 mL of ice-cold PBS, pH7.4. The monolayers were dissolved in 0.5 mL of PBS, pH 7.4 containing1% sodium dodecyl sulfate for 15 min at room temperature and thencounted for radioactivity using a Packard gamma counter. Protein contentwas determined as described above, and final tabulated values wereexpressed in terms of molecules of EC20 per cell.

As shown in FIG. 6, the cell uptake of ^(99m)Tc-EC20 was found to bedependent on the extracellular concentration. The particular KB cellsused were determined to bind a maximum of four million molecules of thefolate radiopharmaceutical per cell. Scatchard analysis of the dataestimated the K_(D) of binding to be 3.2 nM, a value comparable with theK_(D) observed for the vitamin folate binding to these same cells.

Although the full identity of the Peak B component was not established,UV absorption analysis indicated that it contained a folate moiety(i.e., the absorption spectrum contained folate's signature secondaryabsorption peak at 363 nm). This HPLC-purified radiolabeled material(Peak B material) was collected and then added to cultured KB cells. Asshown in FIG. 7, the cell uptake of the ^(99m)Tc-labeled Peak Bcomponent was also found to be dependent on the extracellularconcentration. Scatchard analysis of the data estimated the K_(D) ofbinding to be 1.1 nM. Interestingly, the cell association of Peak B wascompletely blocked in the presence of excess folic acid, indicating thatthis minor formulation by-product is also capable of targetingFR-positive cells for radiodiagnostic purposes.

EXAMPLE 10

Blood Clearance.

Animals used for this study were maintained on a folate-free diet(Harlan #TD-90261) for approximately three weeks prior to doseadministration. Acclimation to this special diet is essential becauseregular rodent diets contain large amounts of folic acid (6 mg/kg chow)and promote high serum folate levels in mice. Furthermore, previousstudies have shown that mice placed on a folate-free diet for 3 weekshad maintained a safe serum folate level of 25±7 nM, which is slightlyhigher than the 9-14 nM concentration measurable in human serum.

The ^(99m)Tc-EC20 solution was prepared on the day of use and hadinitially contained 100 μg of EC20 per milliliter. The solution wasfurther diluted with sterile saline to prepare working stock solutions.The radiochemical purity of the product was estimated to be ˜94% by TLC.Each animal received a dose of 50 μg/kg EC20 (67 nmol/kg) inapproximately 0.1 mL volume i.v. via the tail vein during brief diethylether anesthesia. At the designated times (see FIG. 8) post-injection,each animal was euthanized by CO₂ asphyxiation, and blood wasimmediately collected by cardiac puncture.

As shown in FIG. 8, ^(99m)Tc-EC20 was rapidly removed from circulationin the Balb/c mouse. The plasma half life of this radiopharmaceuticalwas estimated to be ˜4 minutes, and less than 0.2% of the injected^(99m)Tc-EC20 dose remained in circulation after four hours (assumingthat blood represents 5.5% of the total body mass). This data indicatesthat folate conjugates are rapidly removed from circulation followingintravenous administration, and that valuable tissue biodistributiondata can be obtained after only a few hours post-injection without theconcern for non-specific tissue uptake due to blood-borne radioactivity.

EXAMPLE 11

Tissue Distribution Studies.

The ability of ^(99m)Tc-EC20 to target tumors in vivo was assessed usinga FR-positive M109 model. These tumor cells are syngeneic for the Balb/cmouse, and they reproducibly form subcutaneous solid tumors within twoweeks post inoculation. ^(99m)Tc-EC14, which is structurally similar to^(99m)Tc-EC20 except it contains one additional _(D)-Glu residue (i.e.,Pte-_(D)-Glu-_(D)-Glu-βDpr-Asp-Cys), ^(99m)Tc-EC28 (a non-pteroatecontaining control consisting ofbenzoyl-_(D)-Glu-_(n)-Glu-βDpr-Asp-Cys), and the previously reported¹¹¹In-DTPA-folate radiopharmaceutical were also evaluated in thisbioassay. Importantly, the ^(99m)Tc-EC28 control agent will not bind tocell surface FRs because it lacks an essential pteridine ring moiety.

Four to five week-old mice (Balb/c strain) were purchased from HarlanSprague Dawley, Inc. (Indianapolis, Ind.) and were maintained on afolate-free diet for a total of three weeks prior to the experiment.Syngeneic, FR-positive M109 tumor cells (1×10⁶ per animal) wereinoculated in the subcutis of the right axilla two weeks prior to theexperiment. All mice were females, and the tumor weights were 54.2±29.8mg on the day of this experiment. A stock ^(99m)Tc-EC20 solutioncontaining 100 μg of agent per milliliter was prepared on the day ofuse, and its radiochemical purity was >96%. The two additional^(99m)Tc˜chelating agents, ^(99m)Tc-EC14 and ^(99m)-Tc-EC28 as well as¹¹¹In-DTPA-folate were also prepared to >90% radiochemical purity. Allsolutions were diluted with either saline alone or a saline solutioncontaining 100 equivalents of folic acid (for competition) such that thefinal radiopharmaceutical concentration was 10 μmol/mL.

Animals received an approximate 40 μmol/kg i.v. dose of test article in100 μL volume via a lateral tail vein during brief diethyl etheranesthesia. Four hours post-injection, animals were sacrificed by CO₂asphyxiation, and dissected. Selected tissues were removed, weighed, andcounted to determine ^(99m)Tc distribution. CPM values weredecay-corrected, and results were tabulated as % injected dose per gramof wet weight tissue.

As shown in Table 3 (below), the three “folate” containingradiopharmaceuticals, ^(99m)Tc-EC14, ^(99m)Tc-EC20 and¹¹¹In-DTPA-Folate, predominantly accumulated in the FR-positive tumorand kidneys, however the kidneys concentrated a higher percent injecteddose per gram of tissue (% ID/g) than did the tumor. Interestingly, thenet tumor accumulation of ¹¹¹In-DTPA-Folate and ^(99m)Tc-EC20 was nearlythe same (19 and 17% ID/g, respectively), whereas the tumor uptake of^(99m)Tc-EC14 was somewhat less at ˜10% ID/g. Nonetheless, all threeagents displayed high tumor to blood ratios (>30 to 1). TABLE 3Biodistribution of Folate Radiopharmaceuticals in Balb/c Mice BearingSubcutaneous M109 Tumors. % Injected Dose per Gram Tissue (4 hr postintravenous injection)* ¹¹¹In-DTPA- ^(99m)Tc-EC14 + ^(99m)Tc-EC20 +¹¹¹In-DTPA- Folate + ^(99m)Tc-EC14 Folic acid ^(99m)Tc-EC20 Folic acidFolate Folic acid ^(99m)Tc-EC28 Blood 0.31 ± 0.14 0.19 ± 0.07 0.34 ±0.03 0.09 ± 0.02 0.21 ± 0.10 0.09 ± 0.04 0.06 ± 0.04 Heart 2.39 ± 0.640.08 ± 0.01 1.57 ± 0.26 0.08 ± 0.01 2.57 ± 0.82 0.06 ± 0.02 0.03 ± 0.01Lung 2.08 ± 0.40 0.15 ± 0.04 2.22 ± 0.63 0.31 ± 0.26 1.72 ± 0.61 0.09 ±0.2 0.05 ± 0.01 Liver 3.44 ± 2.19 1.37 ± 0.98 3.56 ± 0.25 1.15 ± 0.225.21 ± 2.63 0.81 ± 0.03 0.50 ± 0.26 Spleen 2.68 ± 2.49 2.99 ± 1.43 0.95± 0.15 0.38 ± 0.33 3.30 ± 2.33 1.46 ± 0.73 0.60 ± 0.38 Intestine 1.70 ±0.55 0.32 ± 0.11 2.56 ± 0.61 2.93 ± 1.49 1.87 ± 0.69 0.82 ± 0.14 0.47 ±0.19 Kidney 98.0 ± 40.7 5.94 ± 0.52 138 ± 12.4 5.64 ± 2.13 191 ± 79.23.14 ± 1.96 0.62 ± 0.14 Muscle 0.99 ± 0.28 0.09 ± 0.11 0.67 ± 0.20 0.06± 0.02 1.19 ± 0.48 0.05 ± 0.04 0.02 ± 0.01 Stomach 1.47 ± 0.58 0.10 ±0.03 1.45 ± 0.55 3.35 ± 5.19 1.62 ± 0.65 0.25 ± 0.20 0.21 ± 0.19 Tumor9.83 ± 2.77 0.43 ± 0.52 17.2 ± 1.02 0.45 ± 0.18 19.3 ± 5.86 0.46 ± 0.420.11 ± 0.06 Tumor/Blood 34.1 ± 7.41 2.00 ± 2.00 51.0 ± 8.20 4.70 ± 1.30102 ± 43.4 5.00 ± 4.60 2.00 ± 0.50*Values shown represent the mean ± s.d. of data from 3 animals.

Folate-specific targeting was further demonstrated by two distinctmethods. First, the accumulation of ^(99m)Tc-EC14, ^(99m)Tc-EC20 and¹¹¹In-DTPA-folate in the FR-positive tumor and kidneys was effectivelyblocked (>94%) when these agents were co-administered with a 100-foldexcess of folic acid. Second, the ^(99m)Tc-EC28 control agent failed toappreciably accumulate in the kidneys and tumor. Both observations showthat an intact “folate-like” (or pteroate) moiety is required to affordtargeted uptake and retention of these radiopharmaceutical agents intoFR-positive tissues.

EXAMPLE 12

Gamma Scintigraphy.

M109 tumor cells (1×10⁶ per animal) were inoculated in the subcutis ofthe right axilla of Balb/c mice two weeks prior to the experiment.Animals received an approximate 50 μmol/kg i.v. dose of test article in100 μL volume via a lateral tail vein during brief diethyl etheranesthesia. Four hours post-injection, animals were sacrificed by CO₂asphyxiation and then placed on top of an image acquisition surface.Whole body image acquisition was performed for 1 minute at a count rateof 50-75,000 counts per minute using a Technicare Omega 500 Sigma 410Radioisotope Gamma Camera. All data were analyzed using a MedasysMS-DOS-based computer equipped with Medasys Pinnacle software.

Uptake of ^(99m)Tc-EC20 by the FR-positive M109 tumors and kidneys wasdemonstrated using this gamma scintigraphy protocol. As shown in FIG. 9,a ventral image of a mouse injected with ^(99m)Tc-EC20 as describedabove localizes the gamma radiation to the two kidneys (K) and the M109tumor mass (T; shoulder region). No appreciable radiotracer was observedin other body tissues. A similar image profile has been reported for the¹¹¹In-DTPA-Folate radiopharmaceutical.

EXAMPLE 13

Urinary Excretion and Metabolism.

The urinary HPLC speciation profile of ^(99m)Tc-EC20 was obtained usingBalb/c mice. Mice (˜20 g each) were injected with 1 mCi (6.7 nmol) of^(99m)Tc-EC20 via a lateral tail vein. Following a 1, 4, or 6 hour timeperiod, groups of two mice were euthanized by CO₂ asphyxiation and urinewas collected. After filtration through a GV13 Millex filter, theradiochemical speciation was assessed using an HPLC system equipped witha Nova-Pak C18 3.9×150 mm column and a radiochemical detector. Thesystem was isocratically eluted with 20% methanol containing 0.1% TFA ata flow rate of 1 mL/minute.

It was previously determined that the primary elimination route for¹¹¹In-DTPA-Folate was via the urine. Similar to the HPLC profile shownin FIG. 2, both the ^(99m)Tc-EC20 standard and the urine samplesexhibited four radioactive peaks. As shown in Table 4 (below), theradiochemical purity of the standard (sum of peaks C and D presumablycorresponding to the syn and anti ^(99m)Tc-EC20) remained constant at˜93% over the 6 hr duration of this experiment. The amount of free^(99m)Tc in the standard (peak A) was ˜2%. Importantly, peak B withinthis radiochemical profile is believed to be EC20 chelated with ^(99m)Tcat an unconventional, less stable position, however the radioactivitymeasured in this fraction was not included in the overall radiochemicalpurity estimation for ^(99m)Tc-EC20. This data collectively indicatesthat the formulation remained stable in saline solution throughout this6 hr investigation.

After 1 and 4 hours post-injection into Balb/C mice, the radiochemicalspeciation profile of ^(99m)Tc-EC20 in the mouse urine did not change.The radioactivity present in the urine at 6 hours post-injection,however, was too low to accurately assay by HPLC. The proportion ofparent drug among radioactive species recovered in urine remainedrelatively constant at approximately 90% throughout the four hoursduring which it could be quantitated. This value is very similar to the93% purity of the standard indicating that ^(99m)Tc-EC20 ispredominately excreted into the urine in an unmodified form. TABLE 4Excretion and Metabolism of ^(99m)Tc-Ec20 from the Balb/c Mouse. Micewere injected with 1 mCi (6.7 nmol) of ^(99m)Tc-EC20 via a lateral tailvein. At the indicated times, groups of two mice were euthanized andurine was collected. The radiochemical speciation was then determined byHPLC. The area percent sum of peaks C and D (syn and anti isomers) isused to calculate the overall purity of intact ^(99m)Tc-EC20. AreaPercent ^(99m)Tc-EC20 Standard Urine Samples (two mice/timepoint) PeakRT (min) 0 hr 1 hr 6 hr 1 hr 4 hr A (pertechnetate) 1.4 2 2.1 1.8 8.36.3 9.4 10.2 B (unknown) 3.4 4.5 4.5 4.8 2.5 2.6 5.4 0 C (isomer 1) 5.515.5 15.7 15.9 20.4 18.1 7.3 11.1 D (isomer 2) 18.5 78 77.7 77.5 68.8 7377.9 78.7 Sum C and D 93.5 93.4 93.4 89.2 91.1 85.2 89.8

EXAMPLE 14

Serum Protein Binding.

Fresh rat serum, and commercial male human serum (type AB donors, SigmaChemical Co.) were used to evaluate in vitro binding of ^(99m)Tc˜EC20 toserum proteins. One minute after ^(99m)Tc-EC20 was mixed with 1 mL ofserum at room temperature, 0.3 mL of the serum solution was transferredto a clean Amicon Centrifree® ultrafiltration device (30,000 NMWL) intriplicate. Within one minute of loading the centrifuge with the serumsolution, the device was spun at 1000×g for 20 minutes at 20° C. 50 μLsamples of the original solution, and of the filtrate from each device,was transferred to a clean tube and counted in an automatic gammacounter. A control solution of ^(99m)Tc˜EC20 mixed with 1 mL of normalsaline was ultrafiltered in an identical fashion. The percentage of free^(99m)Tc was calculated for each of the three samples.

While ^(99m)Tc-EC20 exhibited only a minor level of non-specific bindingto the ultra-filtration device (˜5%), approximately 70% of it was foundto predominantly associate with the >30 kDa serum protein fraction insolutions of rat or human serum (69% and 72%, respectively).Importantly, since ^(99m)Tc-EC20 does effectively and preferentiallyaccumulate within FR-positive tissues (see Table 2 and FIG. 8), itsapparent affinity for serum proteins does not appear to affect thisradiotracer's ability to target FRs in vivo.

EXAMPLE 15

Tissue Distribution Studies.

The protocols used in this example are similar to those described inExample 11. The ability of ^(99m)Tc-EC20 to target tumors in vivo wasfurther assessed using FR-positive M109 and FR-negative 4T1 tumormodels. Six week-old female Balb/c mice (n=3/dose group) were purchasedfrom Harlan Sprague Dawley, Inc. (Indianapolis, Ind.) and weremaintained on a folate-free diet (Harlan TEKLAD) for a total of sevendays prior to tumor cell inoculation.

Syngeneic, FR-positive M109 tumor cells (2×10⁶ P_(o) per animal) orFR-negative 4T1 cells (5×10⁵ P_(o) per animal) were inoculatedsubcutaneously in 100 μl of folate-free RPMI-1640 containing 1%syngeneic mouse serum. A stock ^(99m)Tc-EC20 solution containing 100 μgof agent per milliliter was prepared on the day of use as describedabove.

Sixteen days after tumor cell inoculation, the animals were injectedintravenously with 500 or 1800 nmoles/kg of EC20 for M109 tumor-bearinganimals and 500 nmoles/kg of EC20 for 4T1 tumor-bearing animals (3 miceper dose group). All injections were in 100 μl volumes. Four hourspost-injection, animals were sacrificed by CO₂ asphyxiation, and bloodwas collected by cardiac puncture and the animals were dissected.Selected tissues (heart, lungs, liver, spleen, kidney, intestines,stomach, muscle, and tumor) were removed, weighed, and counted in anautomatic gamma counter to determine ^(99m)Tc distribution. Uptake ofthe radiopharmaceutical in terms of percentage injected dose of wetweight tissue (% ID/g) was calculated by reference to standards preparedfrom dilutions of the injected preparation.

As shown in FIG. 11, folate receptor-specific targeting was demonstratedbecause ^(99m)Tc-EC20 predominantly accumulated in the FR-positive M109tumors and kidneys, and not in the FR-negative 4T1 tumors. Uptake in theFR-negative 4T1 tumors was 7.6-fold lower than in the FR-positive M109tumors. Uptake of ^(99m)Tc-EC20 in normal tissues, except kidney asexpected, was low. These results show that ^(99m)Tc-EC20 targeting isFR-specific.

EXAMPLE 16

Tissue Distribution Studies.

The protocols used in this example are similar to those described inExample 11. The ability of ^(99m)Tc-EC11 (peptide-A₁), ^(99m)Tc-EC13(peptide-A₃), and ^(99m)Tc-EC14 (peptide-A₂) to target tumors in vivowas assessed using the FR-positive KB tumor model. Four week-old malenude mice (n=4/group) were maintained on a folate-free diet for a totalof ten days prior to tumor cell inoculation.

FR-positive KB tumor cells (0.25×10⁶ per animal) were inoculatedsubcutaneously in the intracapsular region. Fourteen days after tumorcell inoculation, the animals (n=4/group) were injected intravenouslywith ^(99m)Tc-EC11, ^(99m)Tc-EC13, or ^(99m)Tc-EC14 at the doses (about12 μg/kg) of the conjugates shown in Table 5 below. Stocks of^(99m)Tc-EC11, ^(99m)Tc-EC13, and ^(99m)Tc-EC14 solutions were preparedon the day of use as described above. About a 20-fold excess of freefolate (about 200 μg/kg) was co-administered to control animals(n=4/group). Four hours post-injection, animals were sacrificed by CO₂asphyxiation, and blood was collected by cardiac puncture and theanimals were dissected. Selected tissues were removed, weighed, andcounted in an automatic gamma counter to determine ^(99m)Tcdistribution. Uptake of the radiopharmaceutical in terms of percentageinjected dose of wet weight tissue (% ID/g) was calculated by referenceto standards prepared from dilutions of the injected preparation.

As shown in Table 5, folate receptor-specific targeting was demonstratedbecause ^(99m)Tc-EC11, ^(99m)Tc-EC13, and ^(99m)Tc-EC14 predominantlyaccumulated in the FR-positive KB tumors and kidneys. The accumulationwas blocked by co-administration of free folate. These results show that^(99m)-Tc-EC11, ^(99m)Tc-EC13, and ^(99m)Tc-EC14 can target tumors invivo in a FR-specific manner.

Similar results (see Table 6 below) were obtained with ^(99m)Tc-EC53(the all D-enantiomer of EC20) using similar protocols except that thedose of ^(99m)Tc-EC53 was about 50 μg/kg and about a 100-fold excess offree folate or cold EC53 was used. As shown in Table 6, folatereceptor-specific targeting was demonstrated because ^(99m)Tc-EC53predominantly accumulated in the FR-positive KB tumors and kidneys. Theaccumulation was blocked by co-administration of free folate. Theseresults show that ^(99m)Tc-EC53 can target tumors in vivo in aFR-specific manner. TABLE 5 Percentage of Injected ^(99m)Tc Dose perGram (Tissue Wet Mass) Peptide A₁-Folate Peptide A₂-Folate PeptideA₃-Folate (EC 11) (EC14) (EC13) Tumor mass (g):  0.112 ± 0.027 0.125 ±0.032  0.160 ± 0.037 0.171 ± 0.044  0.213 ± 0.067 Animal Mass (g): 28.9± 1.3 27.1 ± 1.6  27.6 ± 0.6 27.3 ± 2.7  30.0 ± 1.3 Animal Quantity & 4M4M 4M 4M 4M Gender: Folate-Conjugate 11.9 ± 0.5 13.04 ± 1.12  12.6 ± 0.412.87 ± 1.53  11.7 ± 0.9 Dose (μg/kg): Folic Acid 0 195.1 ± 17.9  0192.6 ± 22.9  0 Dihydrate Dose† (μg/kg): (μmol/kg): 0 0.41 ± 0.04 0 0.40± 0.05 Blood:  0.21 ± 0.01 0.25 ± 0.01  0.19 ± 0.02 0.12 ± 0.02  0.25 ±0.03 Heart:  2.5 ± 0.3 0.36 ± 0.03  3.0 ± 0.5 0.24 ± 0.01  1.6 ± 0.1Lungs:  1.2 ± 0.2 0.35 ± 0.03  1.6 ± 0.3 0.24 ± 0.02  1.1 ± 0.1 Liver &Gall Bladder:  5.4 ± 1.4 1.6 ± 0.1  4.5 ± 1.0 0.66 ± 0.07  3.5 ± 0.7Spleen:  0.38 ± 0.03 0.23 ± 0.01  0.41 ± 0.06 0.15 ± 0.01  0.39 ± 0.06Kidney (one): 67.8 ± 6.9 55.5 ± 2.3  44.2 ± 6.4 20.6 ± 2.4  54.5 ± 2.4Stomach, Intestines &  1.4 ± 0.1 1.1 ± 0.3  1.4 ± 0.1 0.50 ± 0.10  2.5 ±0.2 Contents: Muscle:  1.8 ± 0.1 0.38 ± 0.06  2.4 ± 0.4 0.26 ± 0.02  1.7± 0.2 Tumor:  2.95 ± 0.57 1.47 ± 0.24  5.57 ± 0.76 2.0 ± 0.5  5.46 ±0.45 Tumor/Blood: 13.7 ± 2.1 5.9 ± 0.9 29.3 ± 5.2 17.0 ± 4.5  22.5 ± 1.4Tumor/Liver:  0.57 ± 0.14 0.94 ± 0.09  1.3 ± 0.3 3.0 ± 0.6  1.6 ± 0.2Tumor/Kidney:  0.043 ± 0.005 0.027 ± 0.005  0.13 ± 0.03 0.10 ± 0.01 0.10 ± 0.01 Tumor/Muscle:  1.6 ± 0.3  3.9 ± 0.08  2.4 ± 0.6 7.7 ± 1.7 3.3 ± 0.3 Percentage of Injected ^(99m)Tc Dose per Gram (Tissue WetMass) Peptide A₃-Folate (EC13) HYNIC-Folate Tumor mass (g): 0.0773 ±0.041   0.179 ± 0.060 0.150 ± 0.068 Animal Mass (g): 27.5 ± 1.4  27.5 ±1.4 29.0 ± 2.0  Animal Quantity & 4M 4M 4M Gender: Folate-Conjugate13.03 ± 1.05   1.58 ± 0.03 1.46 ± 0.08 Dose (μg/kg): Folic Acid 191.7 ±15.5  0 169.6 ± 9.4  Dihydrate Dose† (μg/kg): (μmol/kg): 0.40 ± 0.03 00.36 ± 0.02 Blood: 0.16 ± 0.02  0.31 ± 0.06 0.25 ± 0.01 Heart: 0.20 ±0.03  3.4 ± 0.3 0.28 ± 0.02 Lungs: 0.23 ± 0.02  1.2 ± 0.2 0.29 ± 0.03Liver & Gall Bladder: 0.65 ± 0.04  3.9 ± 0.9 0.62 ± 0.02 Spleen: 0.15 ±0.02  0.39 ± 0.12 0.17 ± 0.02 Kidney (one): 29.1 ± 2.2  41.2 ± 8.4 38.4± 2.6  Stomach, Intestines & 3.7 ± 0.7  1.5 ± 0.2 1.2 ± 1.0 Contents:Muscle: 0.21 ± 0.03  2.2 ± 0.3 0.36 ± 0.06 Tumor: 1.67 ± 0.24  4.64 ±0.67 2.31 ± 0.27 Tumor/Blood:  10.2 ± 1.5.  15.1 ± 2.1 9.1 ± 1.0Tumor/Liver: 2.6 ± 0.3  1.3 ± 0.3 3.7 ± 0.5 Tumor/Kidney: 0.058 ± 0.010 0.11 ± 0.01 0.069 ± 0.003 Tumor/Muscle: 8.2 ± 2.0  2.1 ± 0.4 6.5 ± 1.3Values shown represent the mean ± standard deviation.Blood was assumed to account for 5.5% of total body mass.Tumor/Background Tissue ratios based on corresponding % Injected Doseper Gram data†Folic Acid dose co-injected with the ^(99m)Tc-Folate-Conjugate dose.n = 3

TABLE 6 ^(99m)Tc-EC53 ^(99m)Tc-EC53 plus Folic Acid ^(99m)Tc-EC53 plusEC53 Average STD Average STD Average STD Test Article Dose (ug/kg) 50.050.0 50.0 (nmol/kg) 67.0 67.0 67.0 Co-dosed competitor (nmol/kg) 6700.06700.0 Tumor mass (g) 0.2 0.17 0.20 0.14 0.19 0.15 Animals 3F 3F 3Fblood 0.38 0.03 0.244 0.028 0.24 0.06 heart 1.09 0.28 0.15 0.03 0.190.03 lung 0.89 0.30 0.23 0.04 0.26 0.07 liver 3.86 0.96 4.49 1.15 3.770.48 intestine 3.53 0.86 4.33 3.67 3.96 1.86 kidney 77.99 6.19 10.126.91 7.97 1.52 muscle 0.76 0.31 0.11 0.04 0.12 0.06 spleen 0.67 0.220.27 0.06 0.41 0.11 stomach 1.04 0.36 0.30 0.15 0.18 0.01 Tumor 11.774.26 0.53 0.11 1.88 1.56 Tumor/Blood 31.4 13.7 2.2 0.6 4.5 4.8Tumor/Liver 3.4 2.2 0.1 0.0 0.3 0.4 Tumor/Muscle 17.1 7.0 5.2 1.4 8.58.0 Tumor/Kidney 0.2 0.07 0.07 0.05 0.14 0.16Discussion

The invention provides a conjugate of a vitamin and a radionuclidechelator for clinical development as an imaging agent. Exemplary of suchan imaging agent is the newly designed, synthesized, and radiochemicallycharacterized folate-based radionuclide chelator, ^(99m)Tc-EC20.

^(99m)Tc-EC20, a small molecular weight peptide derivative of folatethat contains a _(D)-γ-Glu peptide linkage (see FIG. 1), was synthesizedusing an efficient solid-phase synthetic procedure. In its natural form,folate (or pteroyl-glutamate) has a single glutamyl residue present inan L configuration. However, a _(D)-Glu enantiomer residue wasincorporated into the EC20 molecule. Importantly, similar to EC20,substitution of the _(L)-Glu residue in folic acid with a _(D)-Gluresidue does not alter the ability of folic acid to bind to the highaffinity FR.

EC20 was found to efficiently chelate ^(99m)Tc when in the presence ofα-_(D)-glucoheptonate and tin (II) chloride. When analyzed byradiochemical HPLC, >95% of the resulting ^(99m)Tc-EC20 formulationconsisted of a mixture of syn and anti stereoisomers, each equallycapable of binding to FR with high affinity (see FIG. 3). Approximately3% of the ^(99m)Tc in the formulation was chelated to EC20 at some othersite on the EC20 molecule than the expected Dap-Asp-Cys moiety. Althoughthis component was not isolated in sufficient quantity for optimalcharacterization, it was shown to bind to FR with high affinity (seeFIG. 6). Finally, the remaining 2% of the radioactivity in the^(99m)Tc-EC20 formulation was attributed to free ^(99m)Tc.

^(99m)Tc-EC20 demonstrated both time- and concentration-dependentassociation with FR-positive cells. ^(99m)Tc-EC20 was rapidly clearedfrom the blood (t_(1/2)˜4 min), which is important for diagnosticimaging agents, and ^(99m)Tc-EC20 preferentially accumulated in largeamounts within FR-positive tumors.

The performance of ^(99m)Tc-EC20 was directly compared to that of asimilar FR targeting agent, ¹¹¹In-DTPA-Folate, using two differentmethods. First, both folate-based radiopharmaceuticals were found toequally compete with folic acid for binding to KB FRs (see FIG. 3 andTable 1). Second, the biodistribution of each agent in tumor-bearingmice was nearly identical (see Table 2). High tumor uptake andtumor-to-blood ratios were measured for ^(99m)Tc-EC20. Taken togetherthese results suggest that like ¹¹¹In-DTPA-folate, ^(99m)Tc-EC20 willeffectively localize in FR-positive tumors when clinically administeredto patients.

Several folate-based ^(99m)Tc conjugates have previously been described.Limited biodistribution data is available on a^(99m)Tc-12-amino-3,3,9,9-tetramethyl-5-oxa-4,8 diaza-2,10-dodecanedinoedioxime (OXA) folate conjugate, however moderate levels (˜7% ID/g) oftracer uptake in a KB tumor was reported. Studies involving thebiodistribution of a ^(99m)Tc-ethylenedicysteine˜folate conjugate inmammary tumor-bearing rats were also reported. The rats in that studywere fed a folate-rich diet. Thus, low tumor uptake and lowtumor-to-blood ratios were obtained. Lastly, a^(99m)Tc-6-hydrazinonicotinamido-hydrazido (HYNIC) folate derivative(HYNIC-folate) was shown to accumulate in large amounts within 24JK-FBPtumors. Interestingly, ^(99m)Tc-EC20 accumulated within M109 tumors tonearly identical levels as that of HYNIC-folate in 241K-FBP tumors (˜17%ID/g) (Table 2). These two agents also displayed roughly 50:1tumor-to-blood ratios at 4 hours post intravenous injection.

In summary, a new peptide derivative of folate was created toefficiently chelate ^(99m)Tc. This new compound, ^(99m)Tc-EC20, avidlybinds to FR-positive tumor cells in vitro and in vivo. EC20, was foundto bind cultured folate receptor (FR)-positive tumor cells in both atime- and concentration-dependent manner with very high affinity(K_(D)˜3 nM). Using an in vitro relative affinity assay, EC20 was alsofound to effectively compete with ³H-folic acid for cell binding whenpresented either alone or as a formulated metal chelate. Followingintravenous injection into Balb/c mice, ^(99m)Tc-EC20 was rapidlyremoved from circulation (plasma t_(1/2)˜4 min) and excreted into theurine in a non-metabolized form. Data from gamma scintigraphic andquantitative biodistribution studies performed in M109 tumor-bearingBalb/c mice confirmed that ^(99m)Tc-EC20 predominantly accumulates inFR-positive tumor and kidney tissues. These results show that^(99m)Tc-EC20 is an effective, non-invasive radiodiagnostic imagingagent for the detection of FR-positive tumors. Other EC20-relatedimaging agents were also shown to be effective, including EC11, EC13,EC14, and EC53.

Each year ˜26,000 women in the United States are diagnosed with ovariancancer, and less than 50% of those women survive more than five years.One reason for the low survival rate is the difficulty in diagnosingthis form of cancer. Because of the fear of rupturing an unidentifiedabdominal mass and the potential for spreading cancer throughout theabdominal cavity, fine needle biopsy is not often performed. Rather, thediagnosis and staging of suspicious ovarian masses is typically donethrough surgical laparotomy, which is an invasive and expensiveprocedure. Since ^(99m)Tc-EC20 binds tightly to FR present in largeamounts on ovarian cancers (among others), this radiopharmaceuticalprovides an inexpensive, non-invasive but reliable method for the earlydiagnosis of malignant ovarian cancer. Importantly, ^(99m)Tc-EC20 mayalso help guide the clinical decision process by making possible moredefinitive and earlier diagnosis of recurrent or residual disease.

1. A compound of the formula

wherein V is a vitamin that is a substrate for receptor-mediatedtransmembrane transport in vivo, or a vitamin receptor bindingderivative or analog thereof; L is a divalent linker; R is a side chainof an amino acid of the formula H₂NCHRCOOH; M is a cation of aradionuclide; n is 1 or 0; and k is 1 or
 0. 2. The compound of claim 1wherein V is a vitamin selected from the group consisting of folate,riboflavin, thiamine, vitamin B₁₂, and biotin, or a vitamin receptorbinding derivative or analog thereof.
 3. The compound of claim 1 whereinthe radionuclide is selected from the group consisting of isotopes ofgallium, indium, copper, technetium, and rhenium.
 4. The compound ofclaim 3 wherein the radionuclide is an isotope of technetium.
 5. Thecompound of claim 1 wherein V is folate, or a folate receptor bindinganalog or derivative thereof.
 6. A composition for diagnostic imagingcomprising a compound of the formula

wherein V is a vitamin that is a substrate for receptor-mediatedtransmembrane transport in vivo, or a vitamin receptor bindingderivative or analog thereof; L is a divalent linker; R is a side chainof an amino acid of the formula H₂NCHRCOOH; M is a cation of aradionuclide; n is 1 or 0; and a pharmaceutically acceptable carriertherefor.
 7. The composition of claim 6 wherein V in the compound is avitamin selected from the group consisting of folate, riboflavin,thiamine, vitamin B₁₂, and biotin, or a vitamin receptor bindingderivative or analog thereof.
 8. The composition of claim 6 wherein theradionuclide in the compound is selected from the group consisting ofisotopes of gallium, indium, copper, technetium, and rhenium.
 9. Thecomposition of claim 8 wherein the radionuclide in the compound is anisotope of technetium.
 10. The composition of claim 6 adapted forparenteral administration.
 11. The compound of claim 6 wherein V isfolate, or a folate receptor binding analog or derivative thereof.
 12. Amethod of imaging a population of cells in an animal wherein said cellsare characterized by a vitamin receptor on the surface of said cells,the method comprising the steps of administering to the animal aneffective amount of a composition comprising a compound of the formula

wherein V is the vitamin, or a vitamin receptor binding derivative oranalog thereof, specific for said cell surface vitamin receptor; L is adivalent linker; R is a side chain of an amino acid of the formulaH₂NCHRCOOH; M is a cation of a radionuclide; n is 1 or 0; and apharmaceutically acceptable carrier therefor; and monitoring thebiodistribution of said compound in the animal.
 13. The method of claim12 wherein V in the compound is a vitamin selected from the groupconsisting of folate, riboflavin, thiamine, vitamin B₁₂, and biotin, ora vitamin receptor binding derivative or analog thereof.
 14. The methodof claim 12 wherein the radionuclide in the compound is selected fromthe group consisting of isotopes of gallium, indium, copper, technetium,and rhenium.
 15. The method of claim 14 wherein the radionuclide in thecompound is an isotope of technetium.
 16. The method of claim 12 whereinthe composition is administered parenterally to the patient.
 17. Thecompound of claim 12 wherein V is folate, or a folate receptor bindinganalog or derivative thereof.
 18. A compound of the formula

wherein V is a vitamin that is a substrate for receptor-mediatedtransmembrane transport in vivo, or a vitamin receptor bindingderivative or analog thereof; L is a divalent linker; R is a side chainof an amino acid of the formula H₂NCHRCOOH; M is a cation of aradionuclide; n is 1 or 0; and k is 1 or
 0. 19. A composition fordiagnostic imaging comprising a compound of the formula

wherein V is a vitamin that is a substrate for receptor-mediatedtransmembrane transport in vivo, or a vitamin receptor bindingderivative or analog thereof; L is a divalent linker; R is a side chainof an amino acid of the formula H₂NCHRCOOH; M is a cation of aradionuclide; n is 1 or 0; and a pharmaceutically acceptable carriertherefor.
 20. A method of imaging a population of cells in an animalwherein said cells are characterized by a vitamin receptor on thesurface of said cells, the method comprising the steps of administeringto the animal an effective amount of a composition comprising a compoundof the formula

wherein V is the vitamin, or a vitamin receptor binding derivative oranalog thereof, specific for said cell surface vitamin receptor; L is adivalent linker; R is a side chain of an amino acid of the formulaH₂NCHRCOOH; M is a cation of a radionuclide; n is 1 or 0; and apharmaceutically acceptable carrier therefor; and monitoring thebiodistribution of said compound in the animal.